Synthesis, Crystal Structure, and Unusual Magnetic Properties of InMnO3

Polycrystalline samples of InMnO3, prepared by decomposition of nitrates, were found to be isostructural with LuMuO3 rather than YAIO3 as was the case for flux-grown InMnO3 single crystals. The structure was refined in P63cm from powder neutron data with a = 5.8758(4) Å and c = 11.4715(8) Å at 293 K. Single crystals of ScMnO3 were grown from a PbF2 flux. The structure was refined from single crystal X-ray data also in P63cm with a = 5.8286(6) Å and c = 11.1738(9) Å. Magnetic susceptibility data for InMnO3 show subtle anomalies near 120 and 40 K and a weak maximum at 15 K for applied fields <0.05 T. Neutron diffraction data reveal a complex structure with two distinct classes of reflections. From 295 to 5 K, a broad (100) reflection is seen which is well described by a Warren-type lineshape and which is assigned to two-dimensional in-plane Mn-Mn spin correlations. Below 120 K, reflections indexed as (h 0 n/2) and (h 1 n /2) appear which exhibit an unusual temperature dependence featuring intensity maxima near 60-70 K and which are significantly broader than the resolution limit at all temperatures. These are assigned to correlations between spin-coupled Mn layers, also of short range. The absence of long-range order in InMnO3 is contrasted with the well-established order in isostructural ScMnO3 and LnMnO3. It is argued that the differences can be rationalized in terms of an anomalously long c-axis for InMnO3 which leads to a weakening of key interlayer superexchange pathways necessary for the realization of long-range order. This situation is also discussed in terms of recent mean-field theory for layered antiferromagnets.